In general, a magnetic field is generated when electrical current (or current) flows through a wire. Similarly, a magnetic field is generated when current is drawn from a battery by a connected device. If a wire having current flowing therethrough, or a battery having current drawn therefrom, is too close to a device that is sensitive to magnetic fields (e.g., an electronic compass-magnetometer), it will interfere with the proper function (e.g., orientation) of the device.
As an example, the magnetic fields generated by the flow of electrical current are a problem in the field of remotely piloted vehicles and unmanned aerial systems/drones (UAS). These devices rely heavily on an electronic compass (e.g., a magnetometer) that measures the magnetic field of the earth to determine flight direction. As such, reliable autonomous flight is not possible if the electronic compass is unable to work properly due to magnetic interference.
Since a strong magnetic field is generated by the battery and wiring of an unmanned aerial system during operation, their dependence on an electronic compass is a major safety issue. When the electronic compass experiences magnetic field interference, the UAS can experience what is known as a “fly away”. In this condition, due to the electronic compass being unable to provide accurate direction references, the flight controller attempts to achieve a position in space as instructed by the operator, or other system, but the sensors of the UAS cause it to “fly away” in a random direction. For example, the flight control system may instruct a UAS to fly north. Instead, due to magnetic field interference, the UAS flies south. The “fly away” condition is dangerous because the operator is no longer in control of the UAS and is therefor unable to keep it within the confines of a designated safe operating area. Also, magnetic field interference is a leading cause of UAS crashes.
Lithium chemistry batteries come in two form factors: pouch cells and cylindrical cells. Pouch cells may comprise an anode, a cathode, and an electrolyte packaged in a polyethylene lined aluminum pouch. Cylindrical electrochemical cells may comprise an anode, a cathode, and an electrolyte sealed inside a steel cylinder.
New battery chemistries are most readily available in cylindrical cells (e.g., the 18650 format which is 18 mm in diameter and 65 mm in length) that are standardized and widely available. In general, there are no standardized pouch cells. Thus, obtaining pouch cells with new chemistry typically requires a high volume purchase, the cost of which makes using pouch cells impractical for many applications.
Cylindrical-steel electrochemical cells, as compared to pouch cells, are physically robust and offer better protection from environmental and operational damage. Cylindrical-steel electrochemical cells have low structural failure rates and are able to withstand the impact of a crash. This reduces the likelihood of a battery fire following the crash of a UAS. Unlike pouch cells, cylindrical electrochemical cells do not require special packaging techniques to extract maximum performance and battery life therefrom.
The primary challenge with utilizing cylindrical-steel electrochemical cells in UAS applications is the presence of iron in the steel cylinder. The iron is ferromagnetic and acts as a magnetic core within the current loop (i.e., the supply path and the return path for the electrical current), amplifying the magnetic field strength of the current loop thousands of times over an equivalent current loop having only non-ferrous material therein (e.g., an aluminum/polyethylene pouch cell).
As such, batteries built using cylindrical-steel cells generate large magnetic fields when current is flowing through them. This magnetic field can cause an electronic compass (e.g., a magnetometer) to malfunction if it is positioned near the power source (i.e., one or more cylindrical steel electrochemical cells) of an UAS, making the use of cylindrical-steel electrochemical cells impractical on small/compact UAS.